The HoxA9 transcription factor is of critical interest in human acute myeloid leukemia (AML). HoxA9 is oncogenically activated by multiple chromosome 11 translocations involving MLL1 or Nup98 (e.g. MLLAf9, Nup98-HoxA9). Moreover, signaling through Hox proteins has been linked to leukemia stem cell activity. In vitro, MLL-Af9-initiated leukemias generate colonies in semisolid media, and each colony is able to initiate leukemia upon transplantation and therefore contains at least one leukemia stem cell (LSC). However, because the direct transcriptional targets of leukemic Hox signaling are not well understood, the molecular link between Hox signaling and LSC activity is unknown. Our Preliminary Data indicate that HoxA9 signals through microRNA genes, and that inhibition of these microRNA specifically blocks Hoxmediated transformation (by MLL-Af9 and Nup98-HoxA9) and lowers MLL-Af9 LSC numbers. We hypothesize that these epigenetic components of Hox-leukemic signaling can be exploited to expand our understanding of molecular mechanisms of LSC function, and define new avenues for clinical intervention in AML. To this end, we propose to define biological modifiers that mimic the effect of antagonizing miR function, and to define targets of the relevant microRNA. The proposed research is expected to dissect genes and proteins that sustain the leukemic stem cell phenotype. PUBLIC HEALTH RELEVANCE: Acute myeloid leukemia, a cancer of the blood, accounts for 1.2% of cancer deaths in the United States;however, rates are expected to increase as the population ages. Moreover, research to understand the basis of blood cell transformation should lead to a deeper understanding, and the novel generation, of successful therapeutics for both leukemia and abnormal hematopoiesis.